1. Field of the Invention
The present invention relates to a toner used in electrophotography, image forming methods for visualizing electrostatic charge images, toner jet methods and the like.
2. Description of the Related Art
As image forming methods, many methods such as electrophotography, electrostatic recording, magnetic recording and toner jetting are conventionally known. For example, the electrophotographic method generally involves forming electric latent images on a photosensitive member by any of various units, then developing the latent images with a toner, transferring the toner images to a transfer material such as paper as needed, and thereafter, fixing the toner images on the transfer material by heat, pressure or the like to obtain copies. The remainder of the toner which is not transferred and left on the photosensitive member is cleaned by any of various methods to repeat the above-mentioned process.
In recent years, the security function of copying machines and printers has become important along with the higher resolution and higher performance thereof. For example, prevention of forgery and illegal copying of classified documents, certificates and the like is required and measures against illegal copy by embedded pattern printing and the like are taken.
The embedded pattern printing is a technique in which when documents are printed by a copying machine or printer, an “embedded pattern” concealing character strings is simultaneously printed. In the “embedded pattern”, character strings such as “copying prohibited” are previously embedded so as to be hardly viewed and the embedded pattern has a mechanism in which the character strings stand out on copying by a copying machine. Since the embedded pattern printing utilizes a limit of the reproducible capability of dots in copying by a copying machine, a toner is demanded which is excellent in the density stability (density range), dot reproducibility, and line reproducibility, especially line width reproducibility and tailing occurring in line rear ends.
Further, there are more opportunities to use copying machines and printers also in the fields of information recording due to a present sharp increase in information amount. Typical media for information recording include barcode. However, along with the expanded utilizing fields of the barcode, needs to record more information in smaller spaces increase and the so-called QR code (matrix type two-dimensional code), whereby code information is expressed two-dimensionally, is often used. Since even if the QR code is copied several times by a copying machine, an accurate reading precision is required, a toner excellent in the dot reproducibility and the density stability (density range) is demanded.
As described above, for obtaining the image quality adaptable to embedded pattern printing and QR code printing, image forming methods capable of providing higher resolution, higher speed and longer durability are strongly desired in recent years, and therefore, the toner and others are demanded to be further improved.
For example, with respect to the higher resolution, attempts as measures of the copying machine and printer body sides are made in which the high resolution of latent images is improved by reducing the laser spot diameter and raising the charging uniformity of a photosensitive member. On the other hand, as measures of the toner side, attempts are made in which the reproducibility of latent images is improved, for example, by reducing the particle diameter of a toner for faithfully reproducing latent images having a high resolution. However, also in the case of a toner having a reduced particle diameter, since the toner particles move onto latent images in the state of the aggregating, the merit of reducing the particle diameter of the toner cannot be made the best use of and the improvement in the latent image reproducibility is difficult.
Therefore, for obtaining an image quality of a higher resolution, less cohesion between toner particles is also required.
In high speed development systems, since there is a fear of decrease in the image quality, toner performance adaptable to higher speed is demanded.
Efforts to solve the problems described above by measures from the toner side have been made, but the measures are not sufficient for achieving the high-speed and high resolution image formation demanded in recent years, and leave a room for improvement.
For example, addition of various external additives to toner particles for the purpose of improving the chargeability of the toner and imparting a spacer function is proposed. For example, Japanese Patent Application Laid-Open No. H05-142849, Japanese Patent Application Laid-Open No. H05-224456 and Japanese Patent Application Laid-Open No. 2002-287410 propose that addition of a hydrophobic silica and a spherical silica to toner particles raises the image stability. However, since the particle diameter and addition amount of the spherical silica as well as its interrelation with the toner particles are not fully optimized, embedding and release of the spherical silica easily occur in high speed development system; the toners are not sufficient for achieving the high-speed and high resolution image formation and leave a room for improvement. Further, since the chargeability of the toners is not sufficient, the decrease in the image density and the fogging easily occur during a long duration.
Japanese Patent Application Laid-Open No. 2002-003213 proposes that addition of an amorphous fine silica powder to toner particles improves the chargeability of the toner and raises the image stability. Japanese Patent Application Laid-Open No. 2002-108001 proposes the addition of a fine silica powder manufactured by the so-called sol-gel method to toner particles. Although these methods can surely provide images excellent in the density and the fogging, since pores on the surface of the fine silica powder are not controlled, properties of the surface of the fine silica powder are liable to be nonuniform. The toners adsorb moisture in the air under a high-humidity environment and the charging amount of the toners sometimes decreases. Further, in the case of using the toners for high-speed development systems, since there is a case where the fine silica powder does not sufficiently function as a spacer, the toners have room for improvement in adaptability to the higher speed and higher resolution. Japanese Patent Application Laid-Open No. 2004-334069 and Japanese Patent Application Laid-Open No. H06-019190 propose that addition of titanium oxide and aluminum oxide having been subjected to hydrophobization to toner particles raises the chargeability of the toners. Japanese Patent Application Laid-Open No. 2003-322998 proposes that addition of a fine inorganic powder whose particle diameter is prescribed to toner particles raises the image stability. However, although these also have effects on the improvement in the chargeability of the toner and the stabilization of the image density, since the relation of the fine inorganic powder as additive particles with the toner particles is not optimized, the toners are not sufficient in view of the higher image quality.